1. Field of the Invention
The present invention relates to an inexpensive cutting tool composed of a sintered product of silicon nitride which is suited for cutting cast iron.
2. Description of the Prior Art
Owing to their excellent heat resistance, thermal shock resistance and oxidation resistance, sintered products of silicon nitride have until now been used as engineering ceramics and, particularly, as components for thermal engines such as turbo rotors. Owing to their high toughness and abrasion resistance, furthermore, the sintered products of silicon nitride have also been used as cutting tools.
In recent years, it has been attempted to utilize a sintered product having a high density and a high mechanical strength as a cutting tool as disclosed in Japanese Laid-Open Patent Publications Nos. 32785/1980 and 73670/1981.
Such a sintered product of silicon nitride is usually obtained by adding thereto, as a sintering assistant, a rare earth oxide such as Y.sub.2 O.sub.3 or an aluminum compound such as Al.sub.2 O.sub.3 or AlN followed by firing in a nonoxidizing atmosphere containing nitrogen.
When fired at a high temperature, however, the sintered product of silicon nitride brings about such a problem that the surface of the sintered product is coarsened since silicon nitride is subject to be decomposed. To suppress the decomposition of the silicon nitride, the silicon nitride has heretofore been fired in a pressurized atmosphere of a nitrogen gas.
Even though the silicon nitride can be fired in a nitrogen atmosphere, however, there still remains a problem in that it is not possible to completely suppress the surface from being coarsened. When such a sintered product were to be used as a cutting tool, therefore, the fired surface has to be worked like a mirror surface by removing and polishing the coarsened surface by an amount of not smaller than 100 .mu.m. Besides, the cutting tool having a very complex shape requires a very extended period of time for polishing the surface resulting in an increase in the working cost and an increase in the cost of production.
The sintered product of silicon nitride generally has a Vicker's hardness of about 15 MPa in the mirror surface thereof, and it is desired to further increase the hardness and strength so that it can be used as a cutting tool.